A. Field of the Invention
The device of the present invention generally relates to apparatus for protecting electrical equipment from damage or destruction due to the presence of electrical overvoltage surges, commonly referred to as surge arresters.
B. Description of the Prior Art
A surge arrester is commonly electrically connected across a comparatively expensive piece of electrical equipment to shunt overvoltage surges, for example overvoltage surges due to lightning strokes, to ground to thereby protect the piece of electrical equipment from damage or destruction due to the overvoltage surges.
Such surge arresters include arrester components disposed within an arrester enclosure. The arrester enclosure for an overhead arrester for use in outdoor, contaminated conditions is an insulating housing while the enclosure for a submersible arrester further includes an outer conductive layer or jacket. The arrester components include an arrester element, a connector for connection to a system voltage and a ground connector for connection to a ground potential, and may include a disconnector, A disconnector rapidly extinquishes an electrical arc and/or disconnects the ground lead upon failure of the arrester, as well known in the art. The arrester element includes a valve element and may include a gap element. The valve element has one or more valve blocks each formed of a negative resistance material, silicon carbide and a ceramic binder for example. The gap element has one or more spark gaps typically formed by pairs of opposed conductive gap electrodes separated by gap spacers. For example, a prior art surge arrester is illustrated in U.S. Pat. Nos. 3,727,108 and 3,869,650. For a detailed description of arrester structure and operation, reference may be made to U.S. Pat. No. 3,869,650 which is hereby incorporated by reference herein for all purposes.
Other types of surge arresters utilize a valve element formed as metal oxide varistors and herein termed MOV valve blocks. These arresters do not usually include a gap element in the arrester element. Examples of this type of surge arrester are illustrated in U.S. Pat. Nos. 3,805,114; 3,806,765; and 3,811,103 to which reference may be made for a detailed discussion and which are hereby incorporated by reference herein for all purposes.
As illustrated in the aforementioned patents, prior art surge arresters include a separation between the internal surface of the insulating housing and the outer surfaces of the arrester element; that is, the gap element and/or the valve element.
It is known in the prior art that voltage stresses are present across the separation referred to above which can result in damage or destruction to the elements of the prior art surge arresters. The prior art has attempted to alleviate the voltage stress across the above-mentioned separation by surrounding the separation with an equal potential field. For example, one approach to the problem is the provision of stress relief elements such as the voltage stress relief elements 21 and 24 embedded within the elastomeric housing 1 disclosed in the above-mentioned U.S. Pat. No. 3,727,108.
Prior attempts to eliminate the voltage across such separations normally require a rather complex and expensive construction. In addition, such construction as illustrated in the above-mentioned U.S. Pat. No. 3,727,108 are further complicated when used with the MOV valve blocks illustrated in the above prior art patents.